This invention relates to an electromotive device having a high power-to-weight ratio and being capable for use as a motor, alternator or generator. More particularly, this invention relates to a method for manufacturing such a device which reduces manufacturing complexity and associated manufacturing costs.
Recently, a new type of lightweight electromotive device has been developed which is capable of operating at high efficiency and speeds while maintaining high torque and power. Such devices are described in U.S. Pat. Nos. 4,900,965 and 5,212,419, which are hereby incorporated by reference in their entirety. These patents describe an electromotive device having a large diameter--thin cross section speculation ratio for reducing the weight of the device. The device also has low eddy currents and low opposing induced currents, which increase efficiency, torque and speed of operation.
The devices described in the above-incorporated '965 and '419 patents have low loss and high efficiency because of the incorporation of "I-shaped" stator bars. Such bars include a radially short thickness to reduce stator bar cross leakage. Moreover, losses associated with conductor windings being directly exposed to a magnetic field are also reduced due to the I-shaped bar providing a shield for the magnetic field. Additionally, losses caused by the oversaturation of the stator bars are reduced by designing the rotor to include an amount of permanent magnet material such that the bars would just approach saturation. Lastly, the stator bars are oriented such that their grain orientation is aligned with the magnetic field passing therethrough to further improve efficiency.
As described in the '419 Patent, the above electromotive device has heretofore been manufactured as follows. The I-shaped stator bars are individually stamped from a sheet of stator material and individually stacked together (i.e., laminated) by employing an adhesive such tape or glue prior to winding. The separate stacks of stator bars are then inserted into a special magnetic circular winding fixture which holds the various stacked bars against each other to avoid the formation of any air gaps between bars. After the individual stacks are placed in the fixture, coils of electrical conductors are wound around the stator bars to form the stator assembly. The complete stator assembly is then encapsulated or potted with an epoxy-type resin to fixedly hold the stacks of stator bars in a unitary structure with the stator coils. Such encapsulation is achieved either through vacuum impregnation, dipping, brushing or centrifugal application. The final step of the process includes a high temperature processing step to cure the resin.
Although the electromotive device formed according to the above process provides a device capable of operating at high efficiency and having a high power-to-weight ratio, such a device suffers from a number of drawbacks.
First, the device requires a large number of I-shaped stator bar segments to be individually stamped, handled and stacked during manufacturing. This process can be tedious and expensive. Moreover, manufacturing is further complicated since the individual stacks must be positioned tight against each other to avoid the formation of air gaps between stacks.
Second, in some device configurations, the stacks must be oriented a few degrees from the axis of rotation (e.g., two to six) to reduce any cogging effects. Such spacing and orientation constraints also complicates winding of the bars--which generally must be done by hand in order to maintain proper positioning and orientation among the stacks for high efficiency operation.
Third, the device requires an ,encapsulation step which employs an epoxy-type material to hold the individual stator bars and electrical conductors together to form the stator inductor. Because of such epoxy, the resulting device can suffer from thermal and mechanical degradation. For example, after many hours of operation, the epoxy-encapsulated stator assembly may develope cracks or other defects which can allow moisture to penetrate the epoxy, which may lower the breakdown voltage of the stator. This can result in further degradation of the stator, reducing power output and efficiency of the device. Moreover, the development of cracks in the epoxy could cause the stator to become warped and out of round and, therefore, result in interference between the rotor and stator--further reducing power efficiency and structural integrity of the device.
In light of the above, it would be desirable to be able to provide an improved method of manufacturing a high power-to-weight ratio electromotive device which simplifies manufacturing and reduces associated manufacturing costs.
It would also be desirable to be able to provide such an electromotive device which is less susceptible to thermal and mechanical degradation after extended operation due to the use of material.